Summary of Work: The purpose of the methodology studies undertaken here is to prepare for large and/or definitive projects that examine the role of environmental toxicants in reproductive outcome. Methodology studies may address one part of a difficult investigation (such as validity of measurement), or they may be a pilot to highlight difficulties in a future study to be resolved, or they may be small studies using an accessible agent (rather than a rare environmental exposure) to elucidate mechanisms by which toxins affect reproductive health. This year methodology studies have focused on gestational age at the end of a pregnancy. One project examined the epidemiology of post-term delivery, following reports of elevated rates of post-term infants in polluted areas. Another study examined the best methods of dealing with discrepant gestational age estimates for the same women from different data sets. Methodologic work in reproductive epidemiology using alcohol as a prototypical agent is ongoing.